GNU/Linux has union mount that overlays dirs. So you can mount a writeable dir on top of a read-only dir. When the writeable dir is unmounted the read-only dir is untouched.
I am looking for the same ...

I have a iscsi disk 2TB large. I make backup of that disk every week. The backup script copies the image of the whole iscsi disk into a file on my NFS. Unfortunately, it does not copy the image in one ...